Brushed DC electric motors use a commutator and plurality of brushes for energizing the windings in the armature (rotor) of the motor. These brushes are mechanically biased against the commutator, such as by the action of a compressed spring. Variations in the brush drop, which is the voltage drop due to the brush and brush commutator interaction, as well as resonance of the sprung mass of the brushes in conjunction with rotation of the rotor and other external forces acting on the brushes, introduces current ripple in the motor current that in turn produces torque ripple in the torque output of the motor.
Vehicular electric power steering systems, including column-assist and rack-assist systems, may employ various types of electric motors, depending on the system requirements. Brushed DC electric motors are desirable to provide steering assist in certain electric power steering systems, particularly various column-assist systems where the motor output is mechanically linked to the steering hand wheel used by a vehicle operator to steer the vehicle. Torque ripple in the hand wheel of such steering systems is generally undesirable, since the torque ripple is directly communicated to the vehicle operator. Thus, torque ripple has limited the use of brushed DC motors for electric power steering systems applications, particularly column-assist steering systems. Therefore, it is desirable to reduce torque ripple in brushed DC electric motors to enhance their application and usefulness in electric power steering systems as well as other applications.